In many welding applications, it is necessary to start the arc between a welding torch or electrode and the workpiece without bringing the electrode into contact with, or very close proximity to, the workpiece. This is true in many applications with stringent requirements for high quality welds such as the joining of pipes, boiler tubes and the like in nuclear power plants. Many such precision jobs include specifications which preclude the starting of a weld by bringing the current carrying electrode element substantially into contact with the workpiece. When an arc is started by contact or near contact with the workpiece, the initial arc current is very high and can damage the structural integrity of the workpiece. Hence the requirement in precision welding jobs for non-contact arc starting.
Most such requirements are welding jobs requiring the use of tungsten inert gas (TIG) welding apparatus. However, while metal inert gas (MIG) welding machines are not as commonly used in critical welding applications, it is still desirable in some situations to avoid advancing the consumable wire/electrode of a MIG welding apparatus to the point where it contacts the workpiece in order to strike an arc.
Therefore, prior art machines have been created for starting the arc without electrode/workpiece contact. The essential nature of the arc in an arc welding device is a path of ionized gas between the active electrode and the workpiece, which ionized gas conducts the welding current. It is well known that a very intense electric field must be created between the active electrode or torch and the workpiece to initially ionize the gas and start the arc. Since this must be done with a relatively large distance between the torch and workpiece, the electric field intensity can only be increased by significantly raising the potential difference between the torch and the workpiece.
Most prior art non-contact arc starters have created the necessary potential difference between torch and workpiece by exciting a tuned circuit with a voltage function approximating the mathematical impulse function. This causes the tuned circuit to ring and provide a high voltage oscillatory output. By the nature of the construction of such circuits, they have been physically located physically within or closely adjacent to the welding power supply itself. This is because of the pulse spreading which results from the inability of the welding cable extending from the power supply to the torch to accurately transmit the high frequency components of such an impulse like signal. At the frequencies present in the output of such arc starters, the transmission line effects of the cabling between the welding power supply and the torch become predominant. Thus, the length of cable through which such arc starters can transmit the necessary high voltage to strike an arc is limited. Experience has shown that the length of cable between power supply and torch which causes problems with operating this type of arc starter is shorter than the length at which the resistivity of such cabling begins to cause excessive losses in the cable once the arc is struck.
An additional drawback with this type of prior art arc starter is the fact that, because of its nature, it produces a large amount of radiated radio frequency noise. This noise is transmitted both through the air and through power supply connections among components within the welding power supply. Many modern power supplies depend to a great extent on TTL and other logic circuits for their operation. It is well known to those skilled in the art that the presence of air-borne radio frequency noise of significant any intensity and RF noise on the power supply connections in circuits using this type of device can cause erratic, and potentially destructive, behavior in such circuits.
It is the belief of the inventor of the present invention that at least one arc starter constructed by a competitor of the inventor's assignee has employed a high voltage DC power supply which may be operated to produce a large voltage drop between the torch electrode and a grounded workpiece. The inventor recognizes this as work of another but the description of this device in the background of the invention is not to be taken as a stipulation that such apparatus is necessarily prior art to the present invention.
As best the inventor understands it, this device includes a high current switching diode in series between an active output terminal of the welding power supply, to which the welding torch is normally connected, and a torch connection terminal at the output of the arc starter. The diode is connected so that it is forward biased in the presence of normal welding current through an existing arc, and thus the diode conducts the full welding current once an arc is struck.
A connection of an intermittently operated high voltage power supply is made to the terminal of the diode which causes it to become back-biased when the high voltage signal is applied. Once the arc is struck in response to the intense electric field created by the high voltage power supply, the very low impedance of the welding power supply output and the arc predominate, and the diode becomes forward biased and begins conducting welding current.
The major drawbacks with this type of arc starter are two-fold. First, this type of circuit requires that the diode used in the arc starter conduct the full welding current at all times that welding is taking place. Since high current switching diodes are very expensive devices, this increases the cost of such an arc starter. Since the diode in this circuit must conduct the full welding current, it also increases the probability of failure of the diode.
The operation of this type of diode near its rated current carrying capacity causes the device to be operated near its maximum acceptable junction temperature. Even with the relatively low voltage drop of a forward biased diode, such a device dissipates a considerable amount of heat. Experience has shown that once an arc is extinguished after the diode has been operated near its maximum junction temperature, attempts to restart the arc by reapplication of the high voltage DC signal to one terminal of the diode normally fail until the diode has had sufficient time to cool.
The mechanism which causes this undesirable result is the fact that reverse leakage current in a semiconductor diode increases with junction temperature. The necessary geometry of the junction for such high current devices is such that when they are heated to a point near their maximum rated junction temperature, large leakage currents will flow in the presence of reverse bias. This has the effect of severely diminishing the diode's effectiveness as an ideal diode circuit element. This is particularly true in an arc starter of the type described, since it is normally necessary to apply a voltage to the torch/workpiece gap which exceeds the reverse breakdown voltage of a typical high current switching diode. Therefore, when a reverse bias greater than V.sub.br is applied to a hot diode, very large leakage currents will flow.
In practice, the reverse leakage currents can be so high that the diode no longer effectively "disconnects" the active output terminal of the welding power supply from the point in the circuit at which the high voltage arc starting power supply is connected to one terminal of the diode. This leads to a result in which the high voltage arc starting power supply effectively sees the very low output impedance of the welding power supply in parallel with the impedance which exists between the torch and the workpiece (which is very high in the absence of an arc) and the arc will not start.
Therefore, experience has shown that one using such an arc starter must allow sufficient time, often on the order of five to ten minutes, for the diode of this type of arc starter to cool before an arc can be restarted. This naturally leads to inefficient use of the welding apparatus and the time of the people operating it. This situation in turn translates to increased cost of the welding job, which often finds its way to increased public utility rates in many of the applications for which non-contact arc starters are required.
In summary, the first type of non-contact arc starter described above does not have the problem of a time lag between extinguishing of the arc and the ability of the starter to restart it. Its main drawbacks are the large amount of RF noise such a device produces and the restrictions its use places on the length of the cabling between the welding power supply and the torch. On the other hand, the second type of non-contact arc starter described above overcomes the two main disadvantages of the first type, but has the above-described problems of failure to restart until diode cooling takes place after the arc is extinguished.
Therefore, there is a need in the welding art for a non-contact type arc starter which provides the beneficial results obtained from both types of prior art type arc starters described hereinabove. Furthermore, there is a need in the welding art to provide such an arc starter which will operate dependably with either a DC welding power supply or an AC welding power supply.